The availability of low-cost, low-loss optical fibers has stimulated a veritable revolution in the field of optical communications. Optical transmission has already been realized commercially and such transmission lines now span continents and oceans. Researchers are now blazing the way for the next step in this revolution--the use integrated optical circuits to process information-bearing optical signals. Presently, signals are processed in electrical form, and are transformed to optical form only for the purposes of transmission. Clearly, significant economic advantages will accrue when such signals can be processed while still in optical form, thereby avoiding the need to transform the signals back and forth from electrical to optical form.
The processing of optical signals will necessitate the use of many complex devices such as lasers, detectors, modulators, amplifiers, regenerators, etc. However, commercial use will be realized only when the fabrication of these devices is rendered economically viable through the use of integration techniques, with more than one device fabricated on a single chip. Accordingly, significant research and development effort has been expended exploring the design and manufacture of such integrated semiconductor devices. This application relates to at least one such device--the optical amplifier. (The term optical as used here is meant to include a broader region of the spectrum than simply the visible region. The term is rather related to the developing field of optical communications, and, accordingly, refers to any radiation which can be transmitted through a dielectric medium, usually an optical fiber, with a loss less than 2 dB/km or even 1 dB/km. Currently such radiation extends from approximately 0.5 microns to 20 microns. However, this region may be greater and may be extended in the future. In such event, the term optical is meant to include such an extended region of the electromagnetic spectrum.)
An optical amplifier is a device which receives an optical signal and amplifies it--preferably with minimal distortion or other alteration. Current efforts often involve semiconductor laser structures which are altered, at least to the extent that the laser reflecting surface is treated with an antireflection coating. In such devices the product of the gain and the reflectivity is less than one so that the device does not oscillate. Rather, the device is used to amplify an incoming optical as it passes through the device. Such devices are usually called traveling wave amplifiers, connoting the fact that the optical signal does not pass back and forth within the device, but merely passes through it essentially only once.
A critical consideration in the design of such optical amplifiers involves the stability of the amplifier gain. Clearly any variation in the gain of the amplifier will result in distortion of the signal upon amplification. However, the amplification mechanism, which is relied upon in such devices, may introduce an inherent variation in the device gain. This can be understood by recalling that such devices rely upon the phenomenon of stimulated emission to provide the necessary amplification. Stimulated emission, in turn, requires the establishment of a population inversion, which is depleted every time an optical signal passes through the amplifier, and which is reestablished only over some finite period of time. (In typical semiconductor amplifiers or lasers a population inversion is evidenced by the presence of a specified carrier density.) Consequently, the gain of the amplifier will be reduced for some period of time following the passage of any signal through the amplifier--a time period commonly denoted "the amplifier gain-recovery time".